linux-kernel - Re: [PATCH 1/5] all: s390: move wrapper infrastructure to generic headers

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Date:	Wed, 3 Feb 2016 09:01:59 +0100
From:	Heiko Carstens <heiko.carstens@...ibm.com>
To:	Yury Norov <ynorov@...iumnetworks.com>
Cc:	linux-arch@...r.kernel.org, linux-s390@...r.kernel.org,
	arnd@...db.de, pinskia@...il.com, Prasun.Kapoor@...iumnetworks.com,
	catalin.marinas@....com, Nathan_Lynch@...tor.com,
	linux-kernel@...r.kernel.org, agraf@...e.de,
	klimov.linux@...il.com, broonie@...nel.org,
	joseph@...esourcery.com, schwidefsky@...ibm.com,
	linux-arm-kernel@...ts.infradead.org,
	christoph.muellner@...obroma-systems.com
Subject: Re: [PATCH 1/5] all: s390: move wrapper infrastructure to generic
 headers

On Tue, Feb 02, 2016 at 11:41:56PM +0300, Yury Norov wrote:
> On Tue, Feb 02, 2016 at 08:54:34PM +0100, Heiko Carstens wrote:
> > So I think I can summarize my point to: if you can enforce correctness, why
> > shouldn't you do it if the performance impact is only a single instruction.
> 
> For aarch64 it's 5 instructions. But what's more important (if ever),
> another wrapper takes another i-cache line...
>         <compat_SyS_ftruncate>:
>         stp     x29, x30, [sp,#-16]!
>         mov     x29, sp
>         bl      d40 <do_sys_ftruncate.constprop.3>
>         ldp     x29, x30, [sp],#16
>         ret

Why does gcc allocate a stackframe here? Don't you have tail call
optimization?